This invention relates to a method of making glass articles by the flame hydrolysis technique and, more particularly, to an improved method of making glass optical waveguide filaments.
It has been known for some time that light can be propagated along a transparent filamentary structure having a refractive index that is greater than that of its surroundings, and clad filaments have heretofore been employed to transmit light over relatively short distances. The numerical aperture (NA) of such a filament which is a measure of the light gathering ability thereof, can be approximated by: EQU NA=.sqroot.2n.sup.2 .DELTA. (1)
where n is the average of the core and cladding refractive indices, which are designated n.sub.1 and n.sub.2, respectively, and .DELTA. is the refractive index difference between the core and cladding and is given by the equation, .DELTA.=(n.sub.1.sup.2 -n.sub.2.sup.2)/2n.sub.1.sup.2. In conventional optical filaments, .DELTA. is made quite large so that the NA is large, and therefore, the filament is capable of gathering a relatively large amount of light emitted by a source.
Optical waveguides are low loss filaments which have been recently developed as the transmission medium for high capacity optical communication systems. It would be advantageous for optical waveguides to possess high values of NA for the purpose of accepting a large amount of the light radiated from a source connected thereto. Furthermore, optical waveguides are often grouped into cables or bundles to provide redundancy in case of fiber breakage and to transmit a greater amount of the light generated by a source. Attenuation .gamma. due to random fiber bends, which can be caused by the cabling process, is given by the equation: ##EQU1## where c and p are parameters related to the geometry of the random bends and the index gradient and a is the core radius. Examination of equation 2 shows that the distortion loss .gamma. can be reduced inter alia by increasing .DELTA., a factor which will also increase the NA as indicated by equation 1.
The stringent optical requirements placed on the transmission medium to be employed in optical communication systems has negated the use of conventional glass fiber optics, since attenuation therein due to both scattering and impurity absorption is much too high. Thus, unique methods had to be developed for preparing very high purity glasses in filamentary form. Various methods employing the flame hydrolysis technique for forming glass optical waveguide filaments are taught in U.S. Pat. Nos. Re 28,029, 3,711,262; 3,737,293; 3,823,995 and 3,826,560, the latter two patents being directed to the formation of gradient index waveguides. In accordance with one embodiment of the flame hydrolysis process a plurality of constituents in vapor form are entrained in a gaseous medium in predetermined amounts and thereafter are oxidized in a flame to form soot having a predetermined composition. The soot is applied to the surface of a rotating cylindrical mandrel or starting member. After a first layer of soot is deposited to form the core glass, the composition of the soot is changed to form the cladding glass. Heretofore, the soot has been either sintered as deposited, or it has been sintered in a subsequent operation. After the mandrel is removed, the resulting cylindrical hollow draw blank is heated to a temperature at which the material has a low enough viscosity for drawing and is drawn to reduce the diameter thereof until the inner walls of the hollow member collapse. Continued drawing further reduces the diameter until an optical waveguide filament having the desired dimensions is formed. The rate at which fibers can be drawn from the blank is a major factor in determining cost. Draw rate is in turn limited to that rate at which the hole or aperture in the center of the blank can be reliably closed. The need to acid wash the aperture forming walls in flame hydrolysis-produced blanks also increase the cost of producing optical waveguides.
The value .DELTA. has been maintained relatively small in optical waveguides for a number of reasons. The cladding of low loss optical waveguides has usually been formed of a high purity glass such as silica, whereas the core has been formed of the same high purity glass to which a sufficient amount of dopant material has been added to increase the refractive index of the core to a value greater than that of the cladding. Optical waveguides initially formed by the flame hydrolysis process employed multivalent metal oxides such as titanium oxide, tantalum oxide and the like as the dopant material. An inherent problem of such waveguides was the limitation of dopant material to no more than about 15 wt.%. Although fused silica has excellent light transmission qualities in that absorption and intrinsic scattering of light thereby is exceptionally low, the addition of an excessive amount of the aforementioned multivalent metal oxide dopant material to increase the refractive index would cause absorption of light energy and intrinsic scattering of light to increase to undesirable levels.
Other oxides such as GeO.sub.2 and P.sub.2 O.sub.5, which have more recently been employed, also have excellent light transmission properties. As indicated in the publications: MacChesney et al. "A New Technique for the Preparation of Low-Loss and Graded-Index Optical Fibers", Proceedings of the IEEE, September, 1974, pp. 1280-1281 and Payne et al. "New Silica-Based Low-Loss Optical Fibre", Electronics Letters, Vol. 10, No. 15, July 25, 1974, pp. 289-290, the refractive indices of these oxides are sufficiently greater than that of SiO.sub.2 so that they can be employed as a dopant to increase the refractive index of SiO.sub.2. However, the numerical apertures of such optical waveguides have been relatively low since only a limited amount of such GeO.sub.2 or P.sub.2 O.sub.5 could be incorporated into the core thereof relativeto the amount of such dopant in the cladding due to a mismatch of core-cladding characteristics such as thermal coefficient of expansion and softening point temperature.
Consider, for example, an attempt to fabricate a GeO.sub.2 doped-SiO.sub.2 optical waveguide by a prior art flame hydrolysis process whereby the mandrel aperture remains in the consolidated glass draw blank. Assume that the optical waveguide should have a numerical aperture of 0.24 based upon such considerations as the type of light sources to be employed and the types of bends to which the waveguide will be subjected. Knowing the cladding material to be employed, the cladding refractive index n.sub.2 is known. For example, if the cladding material is SiO.sub.2 doped with B.sub.2 O.sub.3, n.sub.2 is taken to be about 1.458, the refractive index of silica. Using the well known relationship NA=(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2 the refractive index n.sub.1 of the core is determined to be 1.477. It can be determined that the core glass should consist of about 23 wt.% GeO.sub.2 to provide a binary GeO.sub.2 -SiO.sub.2 glass having a refractive index of 1.477. The expansion coefficient of such a core glass is about 15.times.10.sup.-7 per degree C. A pure SiO.sub.2 cladding should not be employed with a core containing 23 wt.% GeO.sub.2 since the expansion mismatch between the core and cladding would render it difficult to consolidate the soot preform without it breaking. Thus, a borosilicate cladding having a roughly matching thermal expansion coefficient is selected. It can be determined that a cladding glass of 12 wt.% B.sub.2 O.sub.3 and 88 wt.% SiO.sub.2 has an expansion coefficient (25.degree.-700.degree. C.) of about 12.times.10.sup.-7 per degree C., the 3.times.10.sup.-7 per degree C. difference in expansion coefficients being acceptable. For this combination of core and cladding glasses the softening point temperatures for the core and cladding are about 1630.degree. C. and 1410.degree. C., respectively. This mismatch of 220.degree. C. in the softening point temperatures of the core and cladding glasses creates problems in the filament drawing process. Since the core glass has a much higher softening point temperature than the cladding glass, the hole in the draw blank is difficult to close during the filament drawing process, and the core diameter to outside diameter ratio tends to vary in the resultant filament. To prevent this problem the core should have a softening point temperature that is close to or somewhat lower than that of the cladding. A well known technique for lowering the softening point temperature of the core involves the addition of B.sub.2 O.sub.3 to the core glass. Addition of B.sub.2 O.sub.3 however, slightly decreases the refractive index of the core glass and increases the thermal expansion coefficient. To match this increased thermal expansion coefficient of the core glass, more B.sub.2 O.sub.3 must be added to the cladding glass, thereby bringing back the viscosity mismatch problem. Thus, the amount of GeO.sub.2 in the core has had to be limited to a value that would permit consolidation of the soot preform without excessive breakage, and therefore, the higher desired value of NA could not routinely be achieved.